The Harrier/AV8 uses cascade 'vectoring' nozzles.
Whilst I have no intention (at this stage!) to build a VTOL plane I wondered if it would be possible to use this principle to have multiple nozzles on a single EDF but built very light (in Depron?) to keep to a 'scale' size.
First a 'test' duct to work out the best way to actually make it. For simplicity the ducting is square with a double cascade. Each cascade vane is made of 0.015" litho plate.
Two cascade units joined together to form a twin nozzle.
The inlet side.
The complete duct unit weighs just 0.2oz (5.9g)
Coupled to my 'spare' 55mm EDF on a thrust test rig the object is find out what, if anything, actually happens.
A run up on a 3s.
It generate 9.1oz thrust drawing 20A
On the same rig with the complete nozzle assembly removed the fan generate 18oz thrust.
Although this result appears rather disappointing the total area of the nozzles is only 54% of the FSA so some reduction in static thrust would be expected.
The next step is to build a four nozzle test duct with a combined nozzle area equal to the FSA.
The front pair of the four nozzle test duct.
The airflow is divided into 4 quadrants. Cascades in the L&R sections direct the flow outward to further cascades that direct it rearward.
The top and bottom quadrants will be ducted back to the similar rear cascades.
This duct will be built with the possibility that it could be incorporated into a simple airframe and actually flown if, and its quite a big if, it produces a reasonable degree of thrust.
This is still just at a development stage using a spare 55mm fan. I suspect it would work rather better (and be easier to make!) if sized for a 70mm unit.
This looks way cool but, I must admit, I'm not sure what you're trying to accomplish. Can you enlighten me? What constitutes a 'cascade duct' and what do you hope it/they will buy you? I ask in all sincerity.
My ultimate goal is to create a scale Harrier with true scale inlets and exhaust nozzles along the same lines as I did with my Fairey Delta 2 and XB70 but that is still some way off. A simple proof on concept test duct comes first!
A cascade is a method of taking an airflow efficiently around a tight radius corner by using multiple narrow chord vanes.
The internal shape required to combine the top and bottom quadrants is quite complex both to keep a constant cross sectional area and to end up as a rectangle to match the rear duct inlet.
The completed 4 nozzle duct
As in the Harrier the front nozzles are further out than the rear ones.
It is almost a Depron Pegasus!
The duct mounted on the thrust test rig.
Despite splitting the airflow into 4 smaller nozzles it does indeed produce more thrust, 11.9oz as compared to 9.1 with the earlier 2 nozzle version.
I believe this level of thrust is more than adequate to fly so the test duct will be incorporated into a simple 'Harrier like' airframe to get an idea of how it flies.
After some thought I decided a flat 'profile' fuselage would be the simplest way to get the 4 nozzle duct into the air but to something like scale for the ducts it would only have a 17" wing span (the Harrier had small wings!).
The simple solution was to stretch the wings a bit to 27".
The profile fuselage is hollow but 15mm thick with 2mm Depron side skins.
The cut out matches the complete 4 nozzle duct assembly which is simply glued in.
The bare fuselage weighs just 0.75oz (22g)
The fuselage is brought to a fine point just ahead of the EDF inlet.
To achieve the required CofG it appears the only place for the battery will be in between the front and rear ducts, probably as a 'saddle' arrangement with one either side.
The wings next.
The space between the ducts is just about big enough for an 850mAh 3s so giving 1700Mah in total.
The fan takes 20A at full power which is just 12C from the battery so this set up should give a reasonable endurance.
The duct is faired in to reduce drag.
The fairing to the rear is even more important as the air streams are close to the fuselage.
The other side is similar but has also to locate the ESC.
A start on the rest of the 'Harrier'.
The wings on a Harrier are remarkably small. Scaled to the size of the fuselage they would only be 17" span.
As this is really only a test piece I decided to stretch the span to 26" but kept the root chord the same so they not only have a bigger area but a higher aspect ratio (more efficient?) as well.
Very simple and light. 2mm Depron skin with 3 shear webs sized to create the wing section.
When the top skin is glued on 'free hand' it results in a true symmetrical wing section. There are no ribs. The leading edge is a 3mm Depron strip sanded to profile.
The wing is very thin so even a 3.7g micro servo on its side has to be flush with the wing under surface.
The original's ailerons were 'inset' to allow the tip wheels to retract backwards. For simplicity I have taken the aileron right out to the tip.
The saddle batteries between the ducts.
The scale all moving tailplane has scale anhedral.
Probably not necessary but the tailplane is fully mass balanced with 0.8g of lead in the leading edge each side.
The completed profile 'big wing' Harrier.
With anhedral on the wing as well I do wonder if it will happier flying upside down!
The ESC is mounted below the duct with its heat sink facing outwards.
It certainly makes for a very compact installation.
The radio (35mHz!) is mounted just aft of the rear duct to keep it away as far as possible from the motor and ESC.
It weighs 11.6oz and has a measured thrust of 11.9oz so it should fly but I suspect it will be a bit of a handful. We shall see.
Well almost to my surprise it does indeed fly.
Not the easiest thing to control (the anhedral works perfectly - it has no roll stability whatsoever!) but plenty of push.
Being small and white I kept fairly high for obvious reasons so there is not much to see in the edited video.
It needs dual rate ailerons setting. I mostly fly low roll rate glider types with my 35mHz gear I have set both rates to 100% in case I accidentally knock the switch so I will just have to be careful!
Congrats! And great job on the cascade system. As far as the anhedral stability, don't designers use other tricks, such as washout, in some way to keep things stable? I thought I read about it years ago, but those brain cells aren't speaking at the moment!
Watching the duct experiments with interest........
Yeah I need to find an airport to fly that thing at. On paper, it should have ample thrust to fly, but not with much excess. I had somewhat square corners inside the exhaust pipe, that I knew were restrictive. Rounding them as much as possible, and I didn't have much area to do so, gained a good 2oz of thrust if I remember correctly.
I understand the anhedral was used on the Harrier primarily because with its swept high wing layout it would have been too stable as a fighter. I believe the 'Dutch roll' characteristics are also improved and finally it meant the wing tip wheel legs are that much shorter.
After some more flying to explore it characteristics a bit more I intend to build another set of cascades but using a 70mm EDF as it is a bit more efficient so will give a better overall thrust to weight ratio.
I might also try to construct it with circular outer cascades as this would be a prerequisite if any sort of vectoring was ever to be attempted.
We shall see.
Just a quick update.
It flies quite well but too slow in a turn and it enters an uncontrollable spin! Steep nose down with rapid rotation. It does sometimes pull out (by itself?) but mostly it just continues all the way down to impact regardless of any stick movements.
This is a bit of a surprise as all my other planes either refuse to spin at all or stop the instant the stick is centred.
I shall have to try again (it is now getting rather battered) from a really great height so I have time to find out if and how recovery can be made.
Its spin characteristics continued to baffle me.
It did not feel like a conventional spin as it appeared to occur during normal flight and there was no recovery.
By the fourth crash the cascade ducts were beyond repair although the wings, tail and all the electrics were undamaged.
As I counld not be sure the cascades them selves might be the cause I rebuilt it with the EDF in a simple 'straight through' installation.
The motor is carefully faired into the profile fuselage.
The batteries are mounted in pods set low down to clear the exhaust air flow.
The ESC is 'wrapped round' the EDF fan case.
Overall it is slightly lighter at 11.7oz (331g) and of course the fan now delivers virtually its bare static thrust of 18oz (510g)!
It flies as before but doing a diving turn for a fast(ish) pass the complete nose section broke free!
The remainder continued to fly well enough to complete a smooth landing with no further damage.
I had assumed previously that all the nose damage was the result of the impact with the ground but this latest incident showed that in fact the profile fuselage would flex and ultimately 'crease' in the region of the fan cut out so the forward fuselage would act like a rudder, so it was not a spin but actually an uncontrolled spiral dive and with fuselage bent there was no recovery.
Suitably reinforced with balsa cheek plates it now flies really well.
It is alarmingly fast!
The final part of the story - it broke.
Not surprising really as I was doing an outside loop.
Note the LH servo has been pulled out by its wire through the upper wing surface.
This time the nose damage was definitely due to ground impact!
Not such a surprising result really as in its final form is was considerably over powered for what was a very light airframe intended to just test the cascade duct system not to do high g aerobatics!.
Although every thing still works (even that pulled out servo!) the wreckage has been put to one side whilst I figure out what to do next.
As the actual flight time with the cascade ducting installed was severely cut short from a fault that was in no way related to the ducting itself I decided to rebuild the Harrier with a completely new set of ducts but this time to make the outer nozzles circular to get an idea of the fabrication difficulties involved.
In fact quite a bit of the original Harrier would new as only the tail assembly and the starboard wing could be salvaged from the original build.
I also chose to alter the initial ducting slightly. Rather than four equal 'quadrants' I divided the duct immediately aft of the fan into L & R 'segments' with the remaining top and bottom areas free to be ducted to the rear.
Hopefully this would improve the overall duct area to wall surface ratio that should reduce the initial duct losses a little bit.
I am concerned that the circular outlet nozzles will require some difficult to make shapes and may be less efficient than the simple 'square' nozzles on the first set.